Typically, drill strings made up of drill rods are used for deep drilling in order to reach subterranean natural oil and/or gas deposits. At the end of the drill string, a drill head for grindingly comminuting the soil is provided, for instance a roller bit or a diamond bit (PDC bit). The drill rods have a free inner diameter of ca. 51 mm (2 in) to ca. 1.22 m (48 in) and lengths of typically 9.1 m (30 ft) or ca. 14 m (46 ft). The drill string is assembled of joined drill rods. The diameter of the drill rods of the drill string currently used for drilling depends on the drilling depth. The drill rods are secured by joints, so that hundreds of drill rods must be connected with each other to reach depths of thousands of meters. A maximum depth of up to ca. 12,000 m below ground can be attained thereby. At the inlet of the well, a concrete foundation is cast to secure the well. From the well protrudes a portion of the drill string which is connected to a derrick crane or the so-called derrick, respectively, to hold the drill string and possibly also to drive it for example by means of a top drive. During drilling, holes of different size and depth are drilled, into which a respective pipe casing is inserted and a cylinder-ring shaped concrete wall to secure the well is cast, in order to hold the drill rods in position and guide them. Furthermore, the pipe casings serve also to prevent rock material from falling off or to prevent the intrusion of groundwater. Typically, a well consists of multiple pipe strings of different diameters and lengths. In that respect the pipe string diameters decrease from near-surface depths towards greater depths.
During the drilling process, the drill head grindingly comminutes the rock material which is generally below it. Typically, the rock material is pumped along the free cylindrical ring-shaped shaft extending around the drill rods from the end of the well to the inlet of the well. For that purpose, a jetting liquid, typically water/oil with clay and/or barytes, is pumped through the drill rods under high working pressure of up to ca. 2,000 bar (30,000 psi), which issues at the drill head and forces the rock material (upwardly) towards the well inlet. Thereby, the jetting liquid serves for stabilizing the well, cooling and lubricating the drill head, removing rock material and removing the rock material from the well extremity.
Due to mankind's high demand for crude oil and natural gas, the necessity for exploiting deeper and deeper and/or very difficult attainable reserves increases, so that nowadays the extraction of crude oil/natural gas from reserves at depths of 2,000 m to 4,000 m below ground is typical. In particular, drillings on the sea bottom (subsea drilling), which are undertaken from drilling vessels or offshore platforms/drilling islands, are applied to exploit new crude oil and/or natural gas resources. Compared to depth drillings on land, depth drillings on the sea bottom lead to major technical difficulties, because the beginning of the well can already be as deep as 4,500 m (15,000 ft) below sea level. At such a great depth, a direct human access is not possible, so that generally remote-controlled systems must be applied. Those are error-prone and their replacement requires a high expenditure of time. Further, due to the saline seawater and higher pressure conditions prevailing on the sea bottom, the deterioration of mechanical parts which are necessary for the drilling process increases. The mechanical parts are subject to accelerated corrosion and/or wear and tear. Drillings are also undertaken in fresh-water lakes, however, they are less common than depth drillings on the sea bottom and serve mainly for research purposes and not for the exploitation of oil deposits and/or natural gas deposits.
The drilling process and also the operation of a well bear the danger of a blowout, i.e. the uncontrolled ejection of material, like e.g. oil, gas, soil, water, rocks or other material, if for instance a rapid pressure change occurs during the drilling or operation of the well. This occurs particularly during the drilling process when the drill head expands into oil deposits or gas deposits. In order to prevent a blowout which leads to severe ecological damage and waste of resources, it is the practice to use blowout preventers (BOP).
Blowout preventers (BOPs) are known from the prior art and serve for pressure adjustment and for covering a well in the case of a blowout. Typically, a stack made up of different blowout preventers is positioned at the ground-level beginning of the well. The blowout preventer stacks can weigh up to 1000 t and reach heights of up to 20 m. Blowout preventer stacks generally comprise pressure pipes which can exert pressure on the material in the well or relieve pressure from the well in order to regulate the pressure in the well and thereby e.g. permit controlled drilling or exploitation of oil and/or gas from the well. Various kinds of blowout preventer stacks are used during drilling the well and during the exploitation via the well. The blowout preventers for the drilling have a working time of ca. 6 months and are subject to checking after that time. In the case of deep sea drillings, the entire blowout preventer stack must be transported for that purpose from the sea bottom to the sea surface. For extraction, also a simpler construction can be used, for instance a Christmas tree/production tree. Christmas trees have much longer working times of up to 25 years. The arrangement and the number of blowout preventers in a blowout preventer stack determines the maximum drilling depth, because typically an adjusted blowout preventer for each pipe diameter used during drilling is available in the blowout preventer stack.
Blowout preventers can be in the form of ram blowout preventers or annular blowout preventers. Ram blowout preventers typically comprise two oppositely arranged rams, jaws or valves, which are displaceable relative to each other. Annular blowout preventers typically include an annular elastic element, which can have a plurality of ring segments which are possibly reinforced by metal segments and which are displaceable so that they can form a hermetic sealing by their contacting surfaces. Depending on the design, and particularly depending on the kind of jaws, ram blowout preventers can serve for cutting through, sealing or impressing a drill rod of the drill string extending along the axis of the well into the blowout preventer in order to counteract the pressure of the upwardly flowing material from the well. Typically, several blowout preventers are arranged in the blowout preventer stack, whereby blowout preventers arranged closer to the deposit are usually provided to envelope and seal the drill rods, and blowout preventers arranged further away from the deposit are provided for separating the drill string and hermetically sealing the well. Annular blowout preventers can be closed to a variable degree of tightness and are provided to achieve hermetical sealing of the well or around a drill rod. Blowout preventers and further blowout preventer stack components are typically operated and driven by means of hydraulic equipments. For that purpose, a hydraulic fluid is forced under pressure to the blowout preventers, which can actuate the blowout preventers by displacing or compressing the rams and/or annular elastic elements in per se known manner, for example opening or closing them.
A typical blowout preventer stack has on its end facing towards the well a wellhead connector, which serves for hermetically enclosing the topmost pipe casing (standpipe casing), a short portion of which protrudes from the concrete floor of the wellhead, and thereby connecting the blowout preventer stack to the well. For that purpose, the wellhead connector has typically a larger diameter than the standpipe casing and is provided with collet segments arranged at an inner circumference. If the wellhead connector is positioned on the standpipe casing, the collet segments can be forced under pressure against a stack connector, which is positioned at the end of the standpipe casing, in order to provide hermetic sealing. In the case of a blowout preventer stack breakdown or if the blowout preventer stack is to be replaced as a matter of routine, the wellhead connector must be opened so that the blowout preventer stack can be removed from the well and be replaced by a new blowout preventer stack or, in the case of extraction, by a Christmas tree.
Atop the wellhead connector follow one or a plurality of pipe ram blowout preventers for sealing respectively different pipe diameters. The pipe ram blowout preventers have two oppositely arranged rams with recesses which are commensurate with the diameter of a drill rod. If a pipe ram blowout preventer is activated, the oppositely arranged rams are moved towards one another until they sealingly enclose a drill rod with a diameter which is commensurate with the recess. Depending on the drilling depth, a different number of pipe ram blowout preventers are arranged in a stacked manner.
Atop the pipe ram blowout preventers follows a shear ram blowout preventer, which is provided to cut through a drill rod of the drill string. For that purpose, the rams of the shear ram blowout preventers have shearing edges, which can cut through drilling rods in the manner of scissors. Preferably, the shear ram blowout preventer also serves for cutting through the drilling rod and simultaneously sealing the drill rod hole. Usually, however, sealing of the shear ram blowout preventer does not suffice, so that often an annual blowout preventer is additionally arranged on top of it. This serves for hermetically sealing the drill rod hole and/or the entire well.
There follows a further annular blowout preventer, which serves to seal the blowout preventer stack. The upper annular blowout preventer is connected to a Lower Marine Riser Package—LMRP.
In a special case of a blowout preventer on the sea bottom, the annual blowout preventer is followed by a riser connector. This is intended for sealingly connecting a riser. The riser typically comprises pressure-tight steel pipes, into the interior of which the drill string and jetting liquid are directed. The inner diameter of the riser is larger than the diameter of the drill string and is typically ca. 533 mm (21 in).
The Lower Marine Riser Package (LMRP) constitutes yet another division plane of the blowout preventer stack if the riser has to be separated from the blowout preventer stack. This can for instance be the case if the drilling vessel must leave its position, e.g. due to an iceberg drifting towards the drilling vessel. In such case, the well can be sealed by means of the blowout preventer stack. The drilling vessel can, after the Lower Marine Riser Package (LMRP) has been separated, leave its position and, at a later point in time, reconnect the riser to the blowout preventer stack.
The blowout preventer stack may not fail, because not sealing the well on the occasion of a blowout is associated with considerable economical and ecological costs. Therefore, there exist high security requirements on blowout preventer stacks, particularly for drillings on the sea bottom. The use of several redundant supply and safety systems is thus indispensable. Therefore, blowout preventer stacks comprise, besides the blowout preventers, kill lines and choke lines connected to separate lines, which are adapted to inject filling material under high pressure into the well and/or the blowout preventer stack or to reduce the pressure in the blowout preventer stack by discharging material in order to still permit successful sealing of the well in the case of complete or partial breakdown of the blowout preventer.
U.S. Pat. No. 3,667,721 discloses a blowout preventer comprising a sealing element having an elastic sealing means. A plurality of metallic displacement means can be slidably moved against a curved inner surface of the housing in order to bring the sealing element into a sealing position, wherein the sealing means is arranged against an actuating piston. The sealing means can be circumferentially in contact with the curved inner surface of the housing to form a seal. The sealing element can respond to changes of the diameter of components of a drilling string by adjustment of the sealing element.
US 2008/0023917 A1 discloses a seal and a method of manufacturing a seal for a blowout preventer. The seal includes a rigid material insert disposed within an elastomeric body, wherein at least one portion is selectively de-bonded from the elastomeric body. On the rigid material insert which is de-bonded from the elastomeric body, a release agent, like silicone, can be applied. The method comprises generating a finite element analysis seal model, wherein a strain plot is analyzed based on displacement conditions, and wherein subsequently in the finite element analysis at least one portion of the rigid material insert is identified, which is selectively de-bonded from an elastomeric body. The method further comprises the manufacture of the seal with the rigid material insert, that is selectively de-bonded from the elastomeric body.
U.S. Pat. No. 6,719,042 B2 discloses the arrangement of shear rams for shearing an oil riser. The arrangement comprises two slidable rams, which are respectively slidable along different ram axes, one of which has an upper blade and the other has a lower blade. The surfaces of the blades of the rams are closely adjacent as the blades for shearing the oil riser are moved towards one another. A sealing system is positioned within a recess in the upper surface of the lower blade. The sealing system comprises an elastomeric seal and an actuator for sealing the lower planar surface of the upper blade. The actuator is movable relative to the lower blade to put the elastomeric seal under tension.
U.S. Pat. No. 5,655,745 discloses a lightweight hydraulic blowout preventer, comprising a blowout preventer body, hinge plates and two pairs of rams. The blowout preventer body has openings for guiding a drill rod and, perpendicularly thereto, two mutually superposed oppositely arranged guideways each for a respective pair of rams. Two bonnets are respectively secured to the blowout preventer body by means of a small number of connecting bolts, which are, viewed from the ram axis, arranged perpendicularly to one another along a continuous radius or along a single line. The bonnets form guideway extensions, in each of which a ram is operating, respectively. A hydraulic piston of a respective ram is surrounded by a metallic sealing, respectively. The bonnets are arranged on hinge plates. The connecting bolts of the bonnets can be unbolted and permit said bonnets to be pivoted apart from the body by means of the hinge plates.
U.S. Pat. No. 7,300,033 B1 discloses a blowout preventer operator closure system comprising a closure member, a piston rod, an operator housing, a piston, a sleeve and a closure rod. The piston rod is coupled with one end to the closure rod. The operator housing is with coupled one end to a bonnet and with a second end to a head. The piston rod extends through the bonnet into the operator housing and is therein connected to the piston having a body and a flange. The sleeve is helically fixed within a cavity of the piston and, by means of the locking rod, which is rotationally fixed to the head, can be displaced axially relative to the piston. One end of the closure rod extends through the head and can be operated under water outside of the operator housing.
WO 02/36933 A1 discloses a blowout preventer including a shut-off device and a connecting channel. The shut-off device can be transversely displaced with respect to the connecting channel by means of a drive device. The shut-off device comprises two individually or synchronously operable electric motors and a self-locking gear unit. The self-locking gear unit is drivingly connected to the electric motors.